1. Field of the Invention
The present invention relates to an optical signal transmission control apparatus and an optical signal transmission control method that control transmission of optical signals transmitted via a plurality of redundant routes. More particularly, the present invention relates to an optical signal transmission control apparatus and an optical signal transmission control method that can continuously adjust a transmission delay difference between optical signals transmitted through a plurality of redundant routes, while preventing degradation of an optical signal.
2. Description of the Related Art
In the past, in optical communications, an introduction of a wavelength division multiplexing (WDM) technology has considerably expanded transmission capacities of optical fiber lines. In recent years, to provide against failure of the optical fiber line, a method that provides active and reserved redundant optical fiber lines, and switches to the reserved optical fiber line when a failure occurs in the active optical fiber line has been adopted.
Communication interruption time caused when switching from the active to the reserved optical fiber lines is generally defined to be equal to or less than 50 milliseconds. To meet the need of high quality communications that does not allow a communication interruption of data even for one bit, a hitless protection switching technique is used.
In the hitless protection switching technique, optical signals received via active and reserved optical fiber lines are converted into electric signals, and data converted into the electric-signals is temporarily stored in a memory. When a failure occurs in the active optical fiber line, a switching process is performed so that the data of the reserved-system is read out from the memory, instead of the data of the active-system.
In this manner, when data is temporarily stored in a memory, communication can be continued by switching the optical fiber line to be used from the active system to the reserved system, without causing a data error even for one bit, even if a failure occurs in an active optical fiber line. At the same time, a transmission delay difference of data generated between the active optical fiber line and the reserved optical fiber line can be absorbed.
However, there are some problems to the hitless protection switching technique using a memory. More particularly, the storage capacity required for a memory increases drastically in proportion to a product of a maximum value of a transmission delay difference of data and a communication speed. When optical signals with different wavelengths are multiplexed for transmission as in the WDM, a memory needs to be provided for each wavelength. Further, in the hitless protection switching technique, an optical signal needs to be converted into an electric signal, whereby a problem is caused that the scale of the configuration increases along the increase of the number of wavelengths.
Therefore, a development of a technology that performs hitless protection switching using an optical signal as it is has been sought after. However, unlike electric signals, it is difficult to keep optical signals to one place. Therefore, it is difficult to store information in the form of optical signals. Further, because the optical signal has a high propagation speed, even if the transmission delay difference of data generated between the active system and the reserved system is minute, a transmission delay needs to be generated by making an optical signal pass through a very long optical fiber in order to adjust the delay.
For example, a difference in distance between the routes of the active system and the reserved system is generally considered to be equal to or more than 600 kilometers. However, to adjust the transmission delay difference generated by the difference in distance, a waveguide of an optical signal that is equal to or more than 600 kilometers is required. In this case, if a waveguide of 600 kilometers is formed by connecting a plurality of optical fibers of 30 kilometers in length, a transmission delay difference that corresponds to the maximum of 15 kilometers cannot be adjusted.
If the difference of distance is 600 kilometers, the transmission delay time difference between the active system and the reserved system is approximately 3 milliseconds. However, in addition to the transmission delay time difference caused by the difference in distance, a transmission delay time difference is caused by a change in temperature environment where the optical fiber line is being laid. More particularly, the transmission delay time difference is caused by an expansion and a contraction of an optical fiber due to a change in the temperature environment, and is approximately 50 nanoseconds per 100 kilometers (approximately 300 nanoseconds to 600 kilometers).
To adjust the transmission delay time difference generated by a change in the temperature environment, a technology that continuously controls the delay difference of the optical signals is required. As such, a technology that combines an optical wavelength converting circuit and a high-dispersion optical fiber, and adds to the optical signal a delay that corresponds to the wavelength converted by the optical wavelength converting circuit is known (for example, refer to Japanese Patent Application Laid-Open No. H8-146479).
Though the conventional technology mentioned above is designed to continuously control the transmission delay difference of the optical signals, it has a problem that the delayed optical signal cannot be used for hitless protection switching as it is.
More particularly, though the transmission of the optical signal through a high-dispersion optical fiber can generates the delay of the optical signal to eliminate a transmission delay difference between the active system and the reserved system, a waveform of the optical signal is distorted due to wavelength dispersion. This leads to a problem that the hitless protection switching cannot be performed while preventing degradation of the optical signal.
This wavelength dispersion can be compensated by using a high-dispersion optical fiber that has reverse characteristics to the optical dispersion characteristics of the high-dispersion optical fiber. However, in this case, the absorbed transmission delay difference of the optical signal is generated again, thereby making it unsuitable for the hitless protection switching.
Therefore, a development of a technology that can continuously adjust a transmission delay difference between optical signals that are transmitted via active and reserved optical fiber lines, while preventing degradation of the optical signal, has become an important issue.